Issues pulling lookup fields for RETS Update

Hello, we are attempting to pull the lookup fields for Highland Lakes from the Partial Listing Resource for RETS Update purposes. We need to pull the lookups from all 5 classes but are running into problems. Everytime we try to pull the lookup fields one or more of the classes are failing with a 20502 error telling us the update template is invalid. It is never the same class that causes the error. Sometimes 4 of the 5 will work. Sometimes only 1 will work. Thanks.